Board Thread:Pokémon Help/@comment-33868430-20180110233933/@comment-30155146-20180112222311

Mega Aggron should be able to DESTROY fairies, but I'm guessing the opposing mon that you ran up against was primarina, which is Water/Fairy and targets Aggron's comparatively mediocre SPDEF.

As I'm assuming you haven't run into Stealth Rock teams, so yeah, mega charizard.